1. Field of the Invention
This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to an apparatus and method for providing a linear position error signal in a disc drive through the appropriate weighting of selected combinations of servo burst signals.
2. Discussion
Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from a substantially cylindrical actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of an array of permanent magnets. When controlled DC current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the permanent magnets and causes the coil to move relative to the permanent magnets in accordance with the well-known Lorentz relationship. As the coil moves relative to the permanent magnets, the actuator body pivots about the pivot shaft and the heads are moved across the disc surfaces.
Typically, the heads are supported over the discs by actuator slider assemblies which include air-bearing surfaces designed to interact with a thin layer of moving air generated by the rotation of the discs, so that the heads are said to "fly" over the disc surfaces. Generally, the heads write data to a selected data track on the disc surface by selectively magnetizing portions of the data track through the application of a time-varying write current to the head. In order to subsequently read back the data stored on the data track, the head detects flux transitions in the magnetic fields of the data track and converts these to a signal which is decoded by read channel circuitry of the disc drive.
Control of the position of the heads is typically achieved with a closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907 entitled HARD DISC DRIVE WITH IMPROVED SERVO SYSTEM, issued to Duffy et al., assigned to the assignee of the present invention and incorporated herein by reference. In such a system, head position (servo) information is provided to the discs to detect and control the position of the heads. As will be recognized, a dedicated servo system entails the dedication of one entire surface of one of the discs to servo information, with the remaining disc surfaces being used for the storage of user data. Alternatively, an embedded servo system involves interleaving the servo information with the user data on each of the surfaces of the discs so that both servo information and user data is read by each of the heads.
With either a dedicated or embedded servo system, it is common to generate a servo position error signal (PES) which is indicative of the position of the head with respect to the center of a selected track. More particularly, during track following in which the head is caused to follow a selected track, the servo system generates the PES from the received servo information and then uses the PES to generate a correction signal which is provided to a power amplifier to control the amount of current through the actuator coil, in order to adjust the position of the head accordingly.
Typically, the PES is presented as a position dependent signal having a magnitude generally indicative of the relative distance between the head and the center of a track and a polarity indicative of the direction of the head with respect to the track center. Thus, it is common for the PES to have normalized values ranging from, for example -1.0 to +1.0 as the head is swept across the track and to have a value of 0 when the head is positioned over the center of the track. It will be recognized that the PES is generated by the servo system by comparing the relative signal strengths of burst signals generated from precisely located magnetized fields in the servo information on the disc surface.
As discussed more fully in the previously incorporated Duffy et al. U.S. Pat. No. 5,262,907, the servo fields are generally arranged in an "offset checkerboard" pattern so that, through manipulation of the magnitudes of the burst signals provided to the servo system as the servo fields are read, the relative position of the head to a particular track center can be determined (and subsequently controlled). More particularly, digital representations of the analog burst signals are typically provided to a servo loop microprocessor, which obtains a digital representation of the value of the PES from a selected combination of the input digital representations of the analog burst signals. The microprocessor then compares the value of the PES to a desired value (indicative of the desired position of the head to the selected track) and issues a digital correction signal to the power amplifier, which in turn provides an analog current to the actuator coil to adjust the position of the actuator accordingly.
It follows that an important consideration in digital servo systems is accurately determining the relationship between the value of the PES and the corresponding distance the head is from a known position, for example the center of a track, in order to effect accurate control of the head position. Particularly, it is important to provide a nominally linear PES over the width of a track to ensure precise servo control and stability of the servo loop.
However, the continuing trend in the disc drive industry is to develop products with ever increasing areal densities (greater than 1 Gbit/in.sup.2) and decreasing access times (less than 10 ms), which places greater demands on the ability of modern servo systems to control the position of data heads with respect to data tracks. As track densities continue to increase, a significant problem that results is the ability to manufacture nominally identical heads for use in the disc drive. That is, a disc drive design typically includes the selection of a nominal head width as a selected percentage of the total track width, such as, for example from 50% to 90% of the total track width. The servo system is then designed to operate with a head having a width that is equal or near to the selected nominal head width, within an acceptable tolerance.
However, as track densities increase, it is becoming increasingly more difficult to manufacture heads which meet the tolerances required for new disc drive designs. That is, while track densities continue to increase, manufacturing variations in head widths generally remain constant. Thus, it is increasingly more difficult to supply a population of heads for such increased track densities. This is particularly true with MR heads, which accommodate higher bit densities per track over the thin-film heads of the previous generation, but as a result of increased complexity of MR heads as compared to thin-film heads, MR heads are particularly difficult to manufacture to the strict tolerances needed to accomplish the track densities required by disc drive manufacturers. For example, disc drives of the present generation may require heads to have a nominal width of about 90 .mu.in., .+-.10 .mu.in. As a result, head manufacturers have engaged in time consuming and expensive measurement and sorting operations in order to supply heads meeting the tolerances required by the manufacturers of new drives. These costs are passed along to the manufacturers of the drives, and ultimately, to the consumer.
A related problem which occurs as track densities increase is variation in the width of the tracks. Whereas such variations in track width have not been a significant factor in obtaining accurate servo control in previous disc drives having relatively lower track densities, as track densities continue to increase, variations in track width become increasingly significant. Such variations in track width can occur as a result of imperfections in the magnetic media of the discs, or can occur as a result of errors in the servo track writing process during manufacturing.
There is a need, therefore, for an improved approach to generating a PES in a digital servo system of a disc drive which can accommodate ever increasing track densities, while compensating for manufacturing variations in the width of the heads, as well as variations in track width.